This application is based upon and claims priority from Japanese Patent Applications No. 2000-8738, filed Jan. 18, 2000 and No. 2000-385646 filed Dec. 19, 2000.
1. Field of the Invention
The present invention relates to a spark plug provided inside a combustion chamber of an internal combustion engine and, more particularly, relates to an improvement of the bonding of an Ir alloy tip essentially consisting of iridium in a spark plug provided with such a tip at its ground electrode or center electrode.
2. Description of the Related Art
In recent years, to extend the plug service life and improve performance, a spark plug has been proposed, as disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-329668, comprising at the discharge gap an iridium tip comprised of an iridium alloy resistance welded as a spark discharge electrode member to a center electrode and/or ground electrode comprised of a nickel-based alloy essentially consisting of nickel.
Japanese Unexamined Patent Publication (Kokai) No. 11-329668 discloses to interpose, between the center electrode and/or ground electrode comprised of the nickel-based alloy and the iridium tip, a relieving layer comprised of a material having a coefficient of linear expansion in the range between the iridium tip and the nickel-based alloy and having a Young""s modulus of elasticity smaller than the tip comprised of the iridium alloy and nickel-based alloy.
Summarizing the problems to be solved by the invention, even Japanese Unexamined Patent Publication (Kokai) No. 11-329668 cannot ensure sufficient bonding between the tip bonding portion of the center electrode and/or ground electrode and the iridium tip.
An object of the present invention is to provide a spark plug having a sufficient bonding force between a tip bonding portion of a center electrode and/or ground electrode and a tip comprised of an iridium alloy.
According to the present invention, there is provided a spark plug provided with a center electrode; an insulator shell holding the center electrode; a ground electrode fixed at the insulator shell and facing the center electrode across a discharge gap; a tip comprised of an iridium alloy essentially consisting of iridium bonded to the center electrode and/or the ground electrode at facing portions of the electrodes; and a stress relieving layer arranged between the tip and a tip bonding portion of the center electrode and/or ground electrode to which the tip is bonded; the tip bonding portion being comprised of an iron-based alloy; and the relieving layer being comprised of an alloy including platinum having a coefficient of linear expansion between that of the iridium alloy and the iron-based alloy and having a Young""s modulus of elasticity smaller than the iridium alloy and the iron-based alloy.
Preferably, the tip bonding portion is comprised of an iron-based alloy containing at least 50 wt % of iron, still more preferably an iron-based alloy containing at least 50 wt % of iron and a balance of at least one of chromium and aluminum.
Preferably, the coefficient of linear expansion of the relieving layer at 900xc2x0 C. is 9.2xc3x9710xe2x88x926 to 11.7xc3x9710xe2x88x926 (/xc2x0 C.).
More preferably, the relieving layer contains at least 60 wt % of platinum.
Still more preferably, a Young""s modulus of elasticity of the relieving layer at 900xc2x0 C. is not more than 15xc3x97104 MPa and even more preferably at least 5xc3x97104 MPa.
Still more preferably, a thickness of the relieving layer is at least 0.2 mm and even more preferably not more than 0.6 mm.
Still more preferably, the tip contains at least 50 wt % of iridium and even more preferably contains at least 50 wt % of iridium and contains at least one of rhodium, platinum, ruthenium, palladium, and tungsten added to the iridium.
Preferably, the relieving layer is comprised of a first relieving layer arranged at the tip side and a second relieving layer arranged at the tip bonding portion, the coefficient of linear expansion of the first relieving layer is between the coefficient of linear expansion of the tip and the coefficient of linear expansion of the second relieving layer, and the coefficient of linear expansion of the second relieving layer is between the coefficient of linear expansion of the first relieving layer and the coefficient of linear expansion of the tip bonding portion.
Preferably, the tip is larger in outer circumference than the relieving layer.